Every eukaryotic gene contains regulatory elements driving transcription of that gene. Such regulatory elements include promoters, which are typically positioned immediately upstream of the coding sequence in a gene. Promoters regulate transcription by providing binding sites for transcription factors, which are a part of the transcription machinery. Promoters are commonly used to express proteins in cell culture and in vivo. Many promoters are known and used for expression of proteins in various expression systems. Examples of promoters include cytomegalovirus (CMV) immediate early promoter, Rous sarcoma virus genome large genome long terminal repeats (RSV), Simian Virus 40 (SV40) promoter, interferon gene promoter, metallothionein promoter, and the thymidine kinase promoter and others, e.g., as described in Fernandez et al. (1999) Gene Expression Systems, Academic Press. However, there is still a need in the art to provide promoters that are capable of generating high levels of expression and/or sustain expression for an extended period of time.
β-actin is a structural protein and is usually expressed in all species, from protozoa to eukaryotes, including humans. The human and chicken β-actin promoters have been previously described. The β-actin promoter, in general, shows a more ubiquitous activity than the CMV promoter which is widely used (Xu et al. (2001) Gene 272:149-156). The chicken β-actin promoter was shown to exhibit a higher activity than viral CMV and SV40 promoters but only when it is linked to a CMV enhancer sequence (Xu et al., supra).
The ribosomal protein S21 (rpS21) which is associated with the 40S subunit of the ribosome. The promoter of the human rpS21 gene was previously identified (GenBank® accession No. AJ250907). Similarly to most ribosomal gene promoters, it lacks conventional transcription elements such as the TATA box and CAAT sequence (Smirnova et al. (2000) Bioorg. Khim. 26(5):392-396).